1. Field of the Invention
The invention relates to a backlight unit and method for equalizing brightness of the backlight unit, and more particularly, to a means of utilizing thermal detecting devices and a feedback system to improve the brightness equalization of the backlight unit.
2. Description of the Prior Art
As technology advances, liquid crystal displays (LCDs) have been widely utilized in digital cameras, PDAs, satellite navigation systems, and flat panel televisions. Being one of the key components of the liquid crystal displays, backlight units are optical devices utilized for providing a light source to the liquid crystal displays.
According to the location of the light source, backlight units are typically divided into two major categories: direct-type, in which the light source is generated from directly underneath the display panel, and edge light type, in which the light source generator is located adjacent to the edge of the display panel. The direct-type backlight units are able to provide a higher intensity of light and are thus more suited for large size display panels such as computer monitors and TV panels while the edge light type backlight units are poorly suited for these larger panels.
Please refer to FIG. 1. FIG. 1 is a cross-sectional view of a conventional backlight unit 10. As shown in FIG. 1, the backlight unit 10 includes a housing 12 serving as the supporting platform of the backlight unit 10, a housing 14, a plurality of light generators 16 for generating lights, a reflecting plate 18 disposed on the surface of the housing 12, a light guiding structure 20, such as a diffusing plate, disposed on the opening of the housing 14 for diffusing the light produced by the light source generator 16 upward to the liquid crystal display panel (not shown) and for providing uniformly scattered lights, and a plurality of reflectors 22 embedded on the edge of the housing 14 and the housing 12 for reflecting the light produced by the light source generator 16 along a horizontal direction to the light guiding structure 20. Additionally, an optic film is typically provided above the light guiding structure 20 for enhancing the backlight unit 10 so as to output light beams with uniform luminous intensity to the housing 12, in which the number and the position of the optical film can be varied according to the requirements of the products.
In general, the light source generator 16 in the backlight unit 10 is typically composed of a plurality of light tubes, such as cold cathode fluorescent lamps (CCFL) or external electrode florescent lamps (EEFL). To meet the requirements of high brightness and low weight, the aforementioned light source generator 16 is often installed in a very narrow and small closed chamber. Thus, the heat generated during operation will accumulate rapidly, leading to a high temperature near the light tubes after operating for a period of time. Consequently, the high temperature will interfere with the normal operation of the backlight unit 10 and deteriorate the illumination efficiency of the lamps.
Please refer to FIG. 2. FIG. 2 is a diagram showing the relationship between the brightness of the cold cathode fluorescent lamp and the ambient temperature. As shown in FIG. 2, the brightness of common cold cathode fluorescent lamps utilized for light source generators will only achieve a maximum value under certain ambient temperature. However, the interior of the direct-type backlight unit 10, especially the type utilized for large-scale liquid crystal displays will generate large amount of heat due to long hours of operation and the large quantity of cold cathode fluorescent lamps, such that the interior temperature of typical backlight units will often reach 50° C. or above and seriously decrease the illumination efficiency of the backlight unit and influence the uniformity of the brightness of the liquid crystal display. Additionally, the high temperature within the backlight unit 10 will also decrease the quality of the display, result in a flickering phenomenon in some regions of the display. This has the effect of reducing the life expectancy of the surrounding devices.
Hence, maintaining and adjusting the illumination efficiency of the backlight unit, increasing the uniformity of the brightness of the unit, and at the same time achieving satisfactory heat dissipation and maintaining the life expectancy of surrounding devices has become a critical task.